Parallel data fetching in NextJS - Performance & Optimization

Performance: Parallel data fetching

HIGH IMPACT

This affects page load speed by reducing the time waiting for multiple data sources to respond, improving the Largest Contentful Paint (LCP).

Fetching multiple API endpoints before rendering a page

NextJS

const [data1, data2, data3] = await Promise.all([
  fetch('/api/data1').then(res => res.json()),
  fetch('/api/data2').then(res => res.json()),
  fetch('/api/data3').then(res => res.json())
]);

All fetches run at the same time, reducing total wait to the slowest fetch only.

📈 Performance GainReduces blocking time by up to 3x; improves LCP and user experience.

Fetching multiple API endpoints before rendering a page

NextJS

const data1 = await fetch('/api/data1').then(res => res.json());
const data2 = await fetch('/api/data2').then(res => res.json());
const data3 = await fetch('/api/data3').then(res => res.json());

Each fetch waits for the previous one to finish, causing sequential delays.

📉 Performance CostBlocks rendering for sum of all fetch times; increases LCP significantly.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Sequential fetches	Minimal	1 (after all data)	High (delayed paint)	[X] Bad
Parallel fetches with Promise.all	Minimal	1 (after all data)	Low (faster paint)	[OK] Good

Rendering Pipeline

Parallel data fetching reduces the time spent in the data retrieval phase before rendering, allowing the browser to start layout and paint sooner.

→Data Fetching

→Rendering

→Paint

⚠️ BottleneckData Fetching (waiting for API responses)

Core Web Vital Affected

LCP

This affects page load speed by reducing the time waiting for multiple data sources to respond, improving the Largest Contentful Paint (LCP).

Optimization Tips

1Always fetch independent data sources in parallel to reduce total wait time.

2Use Promise.all to run multiple fetches concurrently in Next.js.

3Avoid sequential awaits for multiple data calls to improve page load speed.

Performance Quiz - 3 Questions

What is the main benefit of fetching data in parallel in Next.js?

A It reduces total waiting time for all data to load.

B It increases the number of DOM nodes.

C It delays the first paint.

D It blocks JavaScript execution longer.

DevTools: Performance

How to check: Record a performance profile while loading the page; look for long gaps in the 'Network' section where fetches happen sequentially versus overlapping.

What to look for: Overlapping network requests indicate parallel fetching; shorter total blocking time improves LCP.